common: Drop bootstage.h from common header

Move this fairly uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/init/board_init.c b/common/init/board_init.c
index f7c8a17..4c4f0ce 100644
--- a/common/init/board_init.c
+++ b/common/init/board_init.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <bootstage.h>
 
 DECLARE_GLOBAL_DATA_PTR;